Starting with sketchy information acquired during the still on-going Fukushima Daiichi disaster, the thyroid doses were estimated, based on the detailed and precise Cs-137 land contamination density maps constructed through a joint effort of MEXT and US DOE/NNEA. The basic methodology has already been benchmarked in the assessment of the radiological consequences of the Chernobyl accident [1].

From the release fraction assessment based on the contamination maps as well as on-site meteorological data obtained and reported by TEPCO’s temporary monitoring cars, the heavy land contamination zone stretching to a NW direction was found induced with the hydrogen explosion which occurred in 1F1 at 15:36 of March 12, only one day after the tsunami. Due to this early iodine release, the shorter half-lived I-133 induced an additional 20% more thyroid doses from I-131 through inhalation.

The results, as presented in Table 7 and Figure 3, predicted rather large thyroid doses, comparable but less than the author’ previous estimation of thyroid doses through inhalation pathway among the public living near the Chernobyl NPP during the disaster. However, age specific doses are more widely spreading among children aged less than 15 years old and not distinctively large among babies, thanks to early restriction of fresh milk.

It should be noted that, above Zone V (300K-1000K/m2 in Cs-137), the estimated thyroid doses with a range of several tens of mGy are already of concern in a possible future outbreak of hypothyroidism. However, if the exposed individuals happened to be staying indoors at the time of plume passage, an order of magnitude of reduction is expected, although the alleviating effects depends on the airtightness of buildings.

Even with these uncertainties, the estimated thyroid dose calls for continued health monitoring of those young residents who happened to be staying in these highly contaminated zones at the time of the plume passage.

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